As a multilayer circuit board fabrication method, a large variety of methods have been designed, and the methods can be classified from several points of view and used properly in accord with their characteristics.
For example, to classify the printed circuit board fabricating methods with focusing on the conductor portion, the methods can fall into two broad general categories: (1) subtractive method in which a conductor is formed on the whole surface and undesired conductor portions are removed by etching while only desired conductor portions leaving and (2) additive method in which conductor patterns are formed only on the desired portions by the use of method such as plating and others.
Recently, considerable high-density wiring has been required for circuit boards used for various electronic equipments, and even when either of the above-mentioned methods is used, it is general practice to form desired conductor patterns using photolithography. This is because fine patterns can be easily formed by the use of photolithography.
The circuit board fabrication technology using photolithography is briefly described as follows.
In the case of the subtractive method, first of all, a substrate with a conductor formed on the surface is prepared, and on the surface, a coating of photosensitive material (resist, e.g. photoresist) is formed. Thereafter, the resist is processed into a desired pattern by exposure and development, and using the pattern as a mask, the substrate surface conductors located at the pattern opening portions are selectively removed by etching, and finally, the desired conductor pattern is obtained by removing the resist.
On the other hand, by the additive method, the photosensitive material (resist) filmed on the substrate surface is processed into desired patterns by exposing and developing, and then, with the pattern used as a mask, a conductor is formed in accordance with the resist opening pattern by a method such as plating and others, and finally, the desired conductor pattern is obtained by removing the resist.
Whether the subtractive method is used or the additive method is used, the above-mentioned methods are characterized in that the photolithography technology using photosensitive S material is utilized. The use of the photolithography technology enables fine high-density circuit patterns to be comparatively easily formed only by preparing photo masks corresponding to the desired patterns.
However, because the above-mentioned conventional technologies utilize photolithography, the technologies have technical problems specific to photolithography as mentioned below.
(1) It takes long time to begin production after completion of designing circuit patterns:
Fabrication of the photo mask used in the photolithography technology can be started only after desired circuit patterns are designed using CAD (Computer Aided Design) and others and after the data is further converted into processing data of the photo mask fabrication apparatus (photo plotter). According to the fact that substrate fabrication cannot be started unless the photo mask is ready, a period from several days up to one week required to fabricate the photo mask after undergoing these complicated processes causes obstacles for shortening of the fabrication lead time.
(2) Photo masks must be fabricated in each time when the design is changed:
Even in the case of minor design changes such as modifying only part of the patterns, the photo mask must be completely newly fabricated, and the substrate fabrication must be suspended in the period each time until refabrication of the photo mask is completed, which generates vain wait time.
Because in analog signal processing circuits, unexpected malfunction is likely to occur due to noise from peripheral circuits, adjustments of patterns after product assembly (cut-and-try) are frequently carried out, and the process of re-fabricating the photo mask per such adjustments constitutes one of the reasons for prolonged period of developments.
(3) Photo masks must be redundantly fabricated for each fabricating site:
The pattern size on the photo mask is, in general, not exactly same as the pattern size on the design drawing but correction specific to a fabricating line is frequently made. Specifically, the pattern size and profile are corrected with performance and accuracy of various kinds of processing apparatus and equipment (for example, parallelism, wavelength, and aberration of light source in photolithography machine, in-plane distribution of film quality and, temperature distribution in film-forming apparatus, and others) taken into account.
Consequently, even when exactly same products (circuit boards) are manufactured, the photo mask cannot be accommodated among a plurality of fabricating sites, and in order to achieve shared production among sites, photo masks must be fabricated at every fabricating site, causing obstacles to flexible production adjustment.
On the other hand, presently, various information equipments including cellular phones, personal computers, and others are distributed to the public, and a tendency for one person to possess one (or more) of these information equipments is much more strongly developed, and a wide variety of functions and performance that support a wide variety of use environment and use condition of each individual have been required. To satisfy these diverse needs, the production system of “mass-producing single specification products” cannot accommodate, and a high-variety small-lot production and a fabricating system that can adjust the production amount and production sites in accord with demand fluctuation are desired.
In order to solve the problems (1) through (3) above, which constitute big obstacles to these market needs, a technology for selectively irradiate the desired portion only of the resist without using photo masks has been proposed and for example, as exemplified in Japanese Patent Application Laid-Open Publication No. 2003-195511 (and its corresponding U.S. Patent Application Publication US 2003/0124463A1), this is called a direct drawing technology, and is a technique to scan-irradiate the resist surface with a laser beam in place of using photo masks (first conventional technology).
Because in the first conventional technology, no photo mask is used, problems described in Paragraphs (1) through (3) can be solved. However, the laser scan apparatus used in this technology requires large-scale equipment with data processing apparatus to quickly process very large amounts of drawing data combined in addition to the laser light source and complicated optical system, and a new problem of high cost and large consumption energy is generated.
In the mean time, in Japanese Patent Application Laid-Open Publication No. 56-66089 and others, a technique to form resist patterns using an ink jet printing technique in place of photolithography represented by exposure and development of photosensitive material is proposed (second conventional technology). The feature of this technique lies in that chemical-resistant resin ink (resist) is directly drawn on a substrate, and a process for applying the resist to the whole surface and a process for removing resist at undesired portions are no longer required.
As a result, by (1) reduction of photoresist consumption amount, (2) non-use of developer, and (3) shortened fabricating time resulting from omission of processes, reduction of the consumption rate of harmful chemical substances can be achieved, too, in addition to the reduction of the fabricating cost. Because the ink jet printing apparatus requires no complicated optical system or no power supply for laser emission, as compared to the laser-scan apparatus used in the first conventional technology, comparatively lower cost and less power consumption can be achieved.
While ink bleed and instability of ink shape are likely to occur in the ink jet printing technology, a countermeasure technique to specify the surface roughness range of the substrate for such phenomena is proposed in Japanese Patent Application Laid-Open Publication No. 8-242060.
In Japanese Patent Application Laid-Open Publication No. 2002-299833, the second conventional technology is further advanced and a method for directly forming conductor patterns is proposed (third conventional technology). The feature of this technology lies in using a liquid-form paste containing superfine metal particles (nanoparticles) that can be sintered at 250° C. or lower and directly drawing desired circuit patterns on a substrate by printing and other methods. By this kind of feature, (1) there is no need to form conductors in advance on a substrate, (2) resist is not at all used, (3) etching and plating processes can be omitted, and other effects can be achieved.
In addition, in Japanese Patent Application Laid-Open Publication No. 2001-167633 (and its corresponding U.S. patent application Publication US 2001/0004477A1), as a measure to improve the conductivity, it is proposed to use a metallic component containing solution made of superfine compound metal particles uniformly dispersed in a medium, which particles consist of a core portion substantially composed of metallic component with whose average particle size ranging from 1 to 10 nm and a cover layer composed of an organic material chemically bonded to the core portion, because there exists a certain limit to the conductivity after film is formed with use of the metal paste with conventionally used metal powders and resin or glass components dispersed in an organic medium in forming a conductive metal thin film on the semiconductor substrate by the immersion method, spray applying method, and other thin-film forming method.